Cancel credential request for authentication challenges with no UI.

If an authentication challenge is received by NetworkServiceClient --
implying that there were no cached credentials or ambient identity
around to respond to the challenge -- the authentication request should
be canceled. A typo resulted in this pending challenge being supplied an
empty username and password instead.

This condition applies to requests that are made in the absence of a
WebContents, and those made with an associated WebContents that went
away prior to the network service notified the client of the
authentication challenge.

Bug: 874790
Change-Id: Ib3c866d1074b2a431a5196614ac3397bf97ea132
Reviewed-on: https://chromium-review.googlesource.com/1252868
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595930}